Coal, biomass residuals, and solid wastes, such as wood waste, municipal solid waste (“MSW”), or refuse derived fuel (“RDF”), are used as fuels to generate electrical power. However, combustion of these fuels also produces various pollutants, such as nitrogen compounds or sulfur compounds, which are believed to be involved in the formation of smog and acid rain. If the fuel includes mercury, the combustion also produces mercury compounds, which have been identified by the Environmental Protection Agency (“EPA”) as a significant toxic pollutant. The pollutants include nitrogen oxide (“NOx”) compounds, such as nitric oxide (“NO”) and nitrogen dioxide (“NO2”), sulfur oxide (“SOx”) compounds, such as sulfur dioxide (“SO2”) and sulfur trioxide (“SO3”), volatile elemental mercury (“Hg°”) and volatile mercuric chloride (“HgCl2”). Air pollutant control legislation, such as the Clean Air Act and the Clear Skies Initiative, regulates emissions of many of these pollutants and is expected to pass in the legislature and become law in United States and other countries. The EPA is currently required to promulgate a mercury emissions limit under the Maximum Achievable Control Technology (“MACT”) provisions of the 1990 Clean Air Act Amendments. Therefore, many powerplants will be required to decrease emissions of these, and other, pollutants.
If the fuel contains sulfur, the sulfur is typically converted to reduced forms of sulfur, such as hydrogen sulfide (“H2S”), carbonyl sulfide (“COS”), and carbon disulfide (“CS2”) upon gasification of fossil fuels, biomass, and waste materials. Nitrogen contained in the fuel is converted to reduced nitrogen compounds, including ammonia (“NH3”), hydrogen cyanide (“HCN”), and nitrogen (“N2”). Most of the mercury entering with the fuel is converted to volatile Hg° and HgCl2. The combustion of fossil fuels and biomass also liberates acid gases, such as hydrochloric acid (“HCl”), sulfuric acid (“H2SO4”), and phosphoric acid (“H3PO4”). These acid gases are corrosive to equipment used in the combustion, such as a combustion device or boiler tubes in a combustor. Therefore, it is desirable to limit the formation of the acid gases or to remove the acid gases close to their point of generation in the combustion device.
Various technologies have been developed to decrease emissions from coal-fired powerplants. Limestone has been used as a sorbent for SOx pollutants, as disclosed in U.S. Pat. No. 3,995,006 to Downs et al., U.S. Pat. No. 5,176,088 to Amrhein et al. (“Amrhein”), and U.S. Pat. No. 6,143,263 to Johnson et al. This technology is known as limestone injection multiple burner (“LIMB”) technology or limestone injection dry scrubbing (“LIDS”) technology. The limestone is injected into a region of a furnace having a temperature of 2,000° F.-2,400° F.
Organic and amine reducing agents, such as ammonia or urea, are used to selectively reduce NOx pollutants, as disclosed in U.S. Pat. No. 3,900,554 to Lyon. This technique is known as selective noncatalytic reduction (“SNCR”). The reducing agent is injected into a furnace at a temperature of from about 975K to about 1375K so that a noncatalytic reaction selectively reduces the NOx to molecular nitrogen (“N2”). The ammonia is injected into a region of the furnace having a temperature of 1,600° F.-2,000° F.
As disclosed in Amrhein, the LIMB and SCNR technologies have been combined to simultaneously removing the NOx pollutants and the SOx pollutants. The limestone is used to absorb the SOx pollutants while the ammonia is used to absorb the NOx pollutants. However, this combination technology is expensive to implement and adds increased complexity to the process.
NOx reburning has also been used to remove the NOx pollutants, as disclosed in U.S. Pat. No. 5,139,755 to Seeker et al. In NOx reburning, the coal is combusted in two stages. In the first stage, a portion of the coal is combusted with a normal amount of air (about 10% excess), producing the NOx pollutants. In the second stage, the remaining portion of the coal is combusted in a fuel-rich environment. Hydrocarbon radicals formed by combustion of the coal react with the NOx pollutants to form N2. Fuel/air staging has also been used to reduce the NOx pollutants. Fuel and air are alternately injected into a combustor to provide a reducing zone where the nitrogen in the fuel is evolved, which promotes the conversion of the nitrogen to N2. The air is injected at a separate location to combust the fuel volatiles and char particles. By staging or alternating the fuel and the air, the local temperature and the mixture of air and fuel are controlled to suppress the formation of the NOx pollutants. Fuel/air staging attempts to prevent NOx formation while NOx reburning promotes NOx reduction and destruction.
To absorb mercury or mercury-containing pollutants, activated carbon is used as a sorbent, as disclosed in U.S. Pat. No. 5,827,352 to Altman et al. and U.S. Pat. No. 6,712,878 to Chang et al. The activated carbon is present as a fixed or fluidized bed or is injected into the flue gas.
Oil shale is a sedimentary rock that includes an inorganic matrix of carbonate, oxide, and silicate compounds impregnated with a polymeric material called kerogen. Kerogen is an organic substance that is insoluble in petroleum solvents. When heated, the kerogen pyrolyzes to produce gas, oil, bitumen, and an organic residue. Pyrolyzing the kerogen is also known as retorting. Oil shale also includes carbonate minerals, such as calcium carbonate, and other hydrocarbon materials, such as paraffins, cycloparaffins, aliphatic and aromatic olefins, one- to eight-ring aromatics, aromatic furans, aromatic thiophenes, hydroxyl-aromatics, dihydroxy aromatics, aromatic pyrroles, and aromatic pyridines. Oil shale is typically co-located with coal and oil and is found in various regions of the western United States, such as in Utah, Colorado, and Wyoming, and in the eastern United States, such as in Virginia and Pennsylvania. Large deposits of oil shale are also found in Canada, Europe, Russia, China, Venezuela, and Morocco. Given the abundance of oil shale throughout the world, its value would be significant if beneficial uses are identified and employed. Oil shale utilization has not been presently appreciated due to the high cost of recovering the kerogen from the shale.
When oil shale containing considerable amounts of calcium carbonate is burned in a direct combustion process, the calcium carbonate undergoes calcination, which is an endothermic reaction in which the calcium carbonate is converted to lime. For each kilogram of calcium carbonate that is calcined, as much as 1.4 MJ to 1.6 MJ (or about 600 British Thermal Units (“BTU”) to 700 BTU per pound mass) of the available heat energy is consumed. This loss of energy translates to a process efficiency penalty when limestone or dolomite is used as an injected sorbent. In the case of oil shale, the kerogen can be oxidized to offset the heat sink associated with carbonate calcination.
To extract energy from the oil shale, the oil shale is heated in a retorting zone of a fluidized bed combustor to a temperature sufficient to release, but not combust, volatile hydrocarbons from the oil shale, as disclosed in U.S. Pat. No. 4,373,454 to Pitrolo et al. The temperature used in the retorting zone provides minimal calcination of the calcium carbonate. The volatile hydrocarbons flow to a combustion zone of the fluidized bed combustor, where the volatile hydrocarbons are combined with excess air and are combusted. Calcination of the calcium carbonate occurs in the combustion zone. During retorting, nitrogen compounds in the oil shale are converted to NOx compounds and are reduced to nitrogen and water or oxygen by the volatile hydrocarbons.
Oil shale has been used to absorb SO2 and HCl in a circulating fluidized bed, as disclosed in “Combustion of Municipal Solid Wastes with Oil Shale in a Circulating Fluidized Bed,” Department of Energy Grant No. DE-FG01-94CE15612, Jun. 6, 1996, Energy-Related Inventions Program Recommendation Number 612, Inventor R. L. Clayson, NIST Evaluator H. Robb, Consultant J. E. Sinor and in “Niche Market Assessment for a Small-Scale Western Oil Shale Project,” J. E. Sinor, Report No. DOE/MC/11076-2759.